Cellular phone systems and portable/mobile user equipments/terminals based on cellular radio communication have evolved in the past years from analogue, narrowband Frequency Division Multiple Access (FDMA) transmission (1st generation (1G) cellular radio communication systems), first to digital, narrowband Frequency and Time Division Multiple Access (FDMA/TDMA) transmission (2nd generation (2G) cellular radio communication systems), and later to digital, broadband Code Division Multiple Access (CDMA) transmission (3rd generation (3G) cellular radio communication systems).
Further evolution, that will include but will not be limited to the adoption of higher data rates, will be based on an improvement of spectral efficiency of transmission systems. However, given the limits of reasonably achievable spectral efficiency, an increase in transmission bandwidth is foreseen for future generations of cellular phone systems.
Accordingly, research is moving towards new generation cellular radio communication systems having a wider transmission bandwidth than 3G cellular radio communication systems, such as for example those known as 3GPP LTE cellular radio communication systems.
In particular, when transmission bandwidth increases, transceivers typically show an increase in their circuit complexity, depending on the type of modulation and multiplexing used. For instance, 3G cellular radio communication systems based on CDMA operate effectively on bandwidths up to several MHz, 10 MHz being often considered an upper limit for the bandwidth of low-cost commercial CDMA equipments using a RAKE receiver or other Single Carrier (SC) receivers with a time-domain equalizer.
When the bandwidth of the transmission systems becomes larger than a few MHz (about 10 MHz), a multi-carrier modulation is often more suitable to keep the transceivers circuit complexity as low as possible.
In particular, Orthogonal Frequency Division Multiplexing (OFDM) has proved to be particularly suited to use with cost-efficient transceivers that process signals in the frequency domain both on the transmitting and receiving sides. More in detail, OFDM is based upon the principle of Frequency Division Multiplexing (FDM), but is implemented as a digital modulation scheme.
Specifically, the bit stream to be transmitted is split into several parallel bit streams, typically dozens to thousands. The available frequency spectrum is divided into several sub-channels, and each low-rate bit stream is transmitted over one sub-channel by modulating a sub-carrier using a standard modulation scheme, for example PSK, QAM, etc. The sub-carrier frequencies are chosen so that the modulated data streams are orthogonal to each other, meaning that cross-talk between the sub-channels is eliminated. This orthogonality occurs when sub-carriers are equally spaced by the symbol rate of a sub-carrier. OFDM exploits frequency spectrum in a particularly efficient way, thanks to the fact that sub-carriers do not interfere reciprocally even if they have partially overlapping spectra. OFDM also allows to largely simplify Multi-Input-Multi-Output (MIMO) receivers compared to SC systems, MIMO receivers being needed as one of fundamental elements to increase the spectral efficiency.
Moreover, in OFDM, transition from frequency to time domain, and vice versa, is typically performed with relatively low-complexity Inverse Fast Fourier Transform (IFFT) and Fast Fourier Transform (FFT) circuits.
The primary advantage of OFDM is its capability to cope with severe channel conditions—for example, multi-path and narrowband interference—without complex equalization filters. Channel equalization is simplified by using many slowly modulated narrowband signals instead of one rapidly modulated wideband signal. 3GPP LTE cellular radio communication systems adopt an OFDM-based physical layer. In particular, 3GPP LTE cellular radio communication systems will have an OFDM-based downlink and a SC-FDMA-based uplink.
Outside the cellular radio communication systems, transceivers have evolved earlier towards large bandwidths. For example WLANs complying with the IEEE802.11 standard family use a 20 MHz channel, and transmit with a 64-subcarrier OFDM modulation. More specifically, in WLANs, transmission is governed by a Medium Access Control (MAC) protocol, called Carrier Sense Multiple Access with Collision Avoidance (CSMA-CA), that avoids transmission when a given frequency channel is already in use. For this reason, inside a given WLAN cell, there is usually no direct co-channel interference between different transceivers. Moreover, in a hot-spot kind of territory coverage, WLAN cells are usually physically separated, so that other-cell interference is largely limited in most cases.
However, in 3GPP LTE cellular radio communication systems, OFDM is expected to work in a very different environment compared to WLANs.
In fact, in a cellular radio communication system, where a continuous radio coverage is required, the signal transmitted by a transceiver station in downlink (DL) or by a terminal or User Equipment (UE) in uplink (UL) can overlap the service area of neighbouring cells. Demands for high spectral efficiency, on the other hand, practically prevent the adoption of high frequency reuse like in 2G cellular radio communication systems, so that it is expected that for example in 3GPP LTE cellular radio communication systems the frequency reuse factor will be low, if not unitary. In 3GPP LTE cellular radio communication systems it is likely that especially at the cell edge very strong co-channel, intra-system interference will be present, substantially lowering user throughput, if not properly mitigated.
An equivalent interference scenario, but in TDD, could arise e.g. in IEEE802.16 and IEEE802.20 networks and similar.
Inter-cell interference can be avoided or mitigated by layer 2 mechanisms, such as Radio Resource Management (RRM) mechanisms (i.e. interference coordination) and intelligent packet scheduling mechanisms, or by layer 1 mechanisms, such as spatial suppression by means of multiple antennas and cancellation based on detection/subtraction of the inter-cell interference. A classification of these mechanisms can be found for example in 3GPP TR 25.814 “Physical layer aspect for evolved. Universal Terrestrial Radio Access (UTRA)” sec. 7.1.2.6.